The present invention relates generally to the field of optical signal processing apparatus, and more specifically to a spatial light modulator of simplified construction and improved performance.
Two-dimensional spatial light modulators are devices which allow control of an optical wavefront for processing or imaging operations. These devices, often referred to as light valves in the literature, have potential for application in large screen display systems as well as in optical data processing systems, including missile guidance and robotic vision systems. Listed below are several articles which describe the construction and/or operation of various embodiments of spatial light modulators.
1. "A Fast Silicon Photoconductor-Based Liquid Crystal Light Valve", P. O. Braatz, K. Chow, U. Efron, J. Grinberg and M. J. Little, IEEE International Electron Devices Meeting, pp 540-543, 1979.
2. "Oblique-cut LiN.sub.b O.sub.3 Microchannel Spatial Light Modulator", C. Warde and J. I. Thakara, Optics Letters, Vol. 7, No. 7, July 1982.
3. "A First-Order Model of a Photo-Activated Liquid Crystal Light Valve", J. D. Michaelson, SPIE Vol. 218, Devices and Systems For Optical Signal Processing, 1980.
4. "LiNbO.sub.3 and LiTaO.sub.3 Microchannel Spatial Light Modulators", C. Warde, A. M. Weiss and A. D. Fisher, SPIE Vol. 218, Devices and Systems for Optical Signal Processing, 1980.
5. "Silicon Liquid Crystal Light Valves: Status and Issues", U. Efron, P. O. Braatz, M. J. Little, R. N. Schwartz and J. Grinberg, Proc. SPIE Vol. 388, Jan. 1983.
6. "Spatial modulation of light in a photosensitive structure composed of a liquid crystal and an insulated gallium arsenide crystal", I. N. Kompanets, A. V. Parfenov and Y. M. Popov, Sov. J. Quantum Electron. 9(8) Aug. 1979, pp 1070-1071.
One basic form of spatial light modulator comprises a photosensitive semiconductor substrate or wafer, a light blocking layer, a dielectric mirror and an electro-optic crystal (which may in some applications be a liquid crystal), arranged in a sandwich-like composite structure, and having a voltage applied thereacross. The photosensitive semiconductor substrate is often a photodiode formed by depositing p-type silicon material on the silicon substrate and applying a reverse biasing potential thereacross. A control (write) illumination impinges on the face of the photodiode while an output (read) illumination makes a double pass through the electro-optic crystal.
The photodiode responds to intensity variations in the control illumination impinging thereon. In the dark, most of the voltage applied across the composite structure appears across the reverse-biased photodiode. The write beam, however, excites carriers in the silicon, which are driven by the internal field to the Si/electro-optic crystal interface. The voltage across the silicon decreases, while the voltage across the electro-optic crystal increases. The read illumination passes through the electro-optic crystal, is reflected off of the dielectric mirror, and again passes through the electro-optic crystal before emerging from the device. Since the diffraction efficiency of the electro-optic crystal is a function of the voltage applied thereacross, (which is a function of the intensity of the write illumination), optical control of the output (read) illumination is achieved.
The fabrication of suitable photodiodes in high resistance silicon, (or other semiconductors, e.g. gallium arsenide), is a difficult art. Such photodiodes must exhibit a high voltage breakdown characteristic and have low leakage current. Leakage currents are known to occur at the semiconductor edges which adversely affect the overall performance of the spatial light modulator.